1. Technical Field
The invention relates to air brake systems for motor vehicles and more particularly to providing an air compressor system in which a vehicle body computer is programmed to learn normal operating ranges for air brake systems, including particularly the compressor cut-in and cut-out pressures.
2. Description of the Problem
Air compressors on trucks can supply pressurized air to air brake systems for the truck, to any trailers pulled by the truck and to other vehicle systems such as air suspension systems. The operation of the air compressor system is critical to truck operation for engaging the brakes for stopping. For this reason the periodic verification of operation of the compressor and air brake system is important for insuring safe operation of the vehicle and has been made the subject of government regulations.
The air compressor on a truck is typically under the control of a governor which triggers compressor operation in response to falling system pressure. The point where compressor operation engages is called the cut-in pressure. The governor further responds to pressure in the system reaching an upper limit at which point it causes the air compressor to discontinue supplying pressurized air. This point is called the cut-out pressure.
Air compressor system performance varies by manufacturer, by age of the system and by its state of repair. Systems differ in the number of compressed air tanks and in the capacity of those tanks. Some vehicles are equipped with air suspensions which make independent demands on compressed air. Leakage rates vary from system to system and still other factors may effect operation. When new or in good repair an air compressor system should operate at close to its optimal levels. With age and deterioration of the system, the system may come to fall short of these optimal operating values. Inspection regimens should detect failure of a system to operate close to original specification.
Inspection regimens must be established to comply with government regulations. These regimens check variables such as compressor governor cut-out, governor cut-in and monitor gauge pressure against a clock to determine charge build time. A representative procedure requires a driver or operator to:    “Check Air Compressor Governor Cut-in and Cut-out Pressures. Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer's specifications.) Run the engine at a fast idle. The air governor should cut-out the air compressor at about the manufacturer's specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut-in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise.”    “Check Rate of Air Pressure Buildup. When the engine is at operating rpm, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. (If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer's specifications.) In single air systems (pre-1975), typical requirements are pressure buildup from 50 to 90 psi within three minutes with the engine at an idle speed of 600–900 rpm.”Making these determinations manually and daily is obviously time consuming. In addition, the pressure gauges used in making the tests are frequently imprecise and difficult to read.
It would be desirable to equip vehicles to determine normal compressor system operating variables notwithstanding differences in air pressurization systems from vehicle to vehicle. Operators should not be tempted to provide, or guess at, manufacturer's specifications in implementing an inspection regimen. The possibilities for operator error should be reduced. A system which automatically compensates for the effects of extraneous systems of compressed air availability would be further advantageous.